The invention relates to encoding video color information.
The colors of a video image may each be uniquely defined by the characteristics of luminance, hue and saturation. The luminance represents the intensity, or brightness, of the color. The hue represents the dominant electromagnetic wavelength of the color, and the saturation represents the degree to which the hue is mixed with white. In accordance with these characteristics, for purposes of encoding color for transmission via a video signal, each color may be represented in YUV color space. The luminance of each color is represented by the Y component, and the hue and saturation of each color are jointly represented by a linear combination of the U and V components.
As shown in FIG. 1, a video signal typically has successive horizontal timing cycles 12. Each horizontal timing cycle 12 has an active region 15 representing the image information for one horizontal scan line on the screen of a television. Within the active region 15, the magnitude of a signal 14 is representative of the time varying Y component (i.e., the luminance) for the scan line. For purposes of encoding the time varying U and V components (i.e., the time varying hue and saturation) into the active region 15, a much smaller magnitude and higher frequency signal, typically referred to as a chroma signal 16, is added to the signal 14. The phase difference between the chroma signal 16 and a subcarrier reference signal 18 of the same frequency represents the hue, and the magnitude of the chroma signal 16 represents the saturation.
The chroma signal 16 is generated by the quadrature amplitude modulation (QAM) of a U signal (representative of the U component) and a V signal (representative of the V component) which may be represented by the following equation: EQU CHROMA=U sin(wt)+V cos(wt),
where w is the radian frequency of the subcarrier reference signal 18.
As shown in FIG. 2, the quadrature amplitude modulation of the U and V signals is typically accomplished by using the phase of the subcarrier reference signal 18 to look up the sine and cosine components of the chroma signal via a sine look-up table 20 and a cosine look-up table 22. A multi-input multiplier 24 multiplies the U signal by the output of the sine look-up table 20, and another multi-input multiplier 25 multiplies the V signal by the output of the cosine look-up table 22. The products of the multipliers 24 and 25 are furnished to an adder 27 which sums the products together to form the chroma signal 16.